Hi mannem
Very basic solution would be to extract 3 points from the 2D bounding box (in x y z coordinates) along with the farthest away point of the point cloud and a point that would define box orientation. Then draw a box and project it to 2D plane. This will however only give you boxes of the same orientation. If you want the boxes aligned with direction of the object, you need to use a separate NN to infer that.
Hope this helps,
Jaka